Variable transconductance amplifying circuit



Dec. 5, 1949 ALMA 2,490,569

VARIABLE TRANSCONDUCTANCE AMPLIFYING CIRCUIT Filed July 20, 1946Patented Dec. 6, 1949 FYIN G CIRCUIT VARIABLE TRAN SOON DUOTAN CE AMPLI-Gerrit Hendrik Petrus Eindhoven, N etherlands, assignor to HartfordNational Bank and Trust Company, Hartford, Conn, as trustee ApplicationJuly 20, 1946,-Serial No. 685,186 In Belgium June 27, 1945 Section 1,Public Law 690, August 8, 1946 Patent expires June 27, 1965 2 Claims.

The copending patent application, U. S. Serial No. 683,710, describes acircuit arrangement for amplifying electrical oscillations by means ofat least two parallel-connected grid-controlled discharge paths havingcharacteristic curves whose mutual conductance varies as a functionvofthe control-grid voltage, This circuit exhibits the feature thatdischarge paths are used, one of which has a comparatively high averagemutual conductance and a comparatively small grid space, whereas theother has a comparatively'low average mutual conductance and acomparatively large grid space, means being provided which bring aboutautomatically that the amplification, with small amplitudes of theoscillations tob'e amplified, exclusively or almost exclusively takesplace by the first mentioned path and, in the case of large amplitudesof these oscillations, exclusively or almost exclusively by thelast-mentioned path.

Automatic changing over from one discharge path to the other can besecured effectively by supplying to the control grids a control voltagewhich depends upon the amplitude of the oscillations to be amplified,this control voltage blocking the first-mentioned discharge path atcoinparatively high values of the oscillations to be amplified and atthe same time affects the potential of an auxiliary electrode located inthe lastmentioned discharge path (i. e. the path having thecomparatively low average mutual conductance and the comparatively largegrid space) in such a manner as to block this path at comparativelysmall values of the oscillations to b amplified.

In a constructional form of the circuit arrangement, which is set outhereinafter, the said 'auXil-' iary electrode is connected through aresistance to the interconnected anodes of the two discharge paths, thecommon anode circuit of these paths including a preferably highresistance having such a value that the discharge path, in which theauxiliary electrode is located, is blocked in the case of smallamplitudes of the oscillations to be amplified.

The present invention has for its object to provide an improvement ofthis circuit arrangement, by which the maximum obtainable amplificationis increased.

The invention is based on the realisation that According to theinvention the saidlauxiliary electrode is connected through a resistanceto a grid (screen grid) which is located in the firstmentioned dischargepath and to which a positive voltage is supplied through a resistance,the

i said resistances bein given such a size that the discharge path,comprising the auxiliary electrode is entirely or almost entirelyblocked in the case of comparatively small amplitude of the oscillationsto be amplified.

."It has been found, in efiect, that for blocking the last-mentioneddischarge path it is not necessary to connect the auxiliary electrode tothe interconnected anodes of the two discharge paths. It turned outthatthe cutting out of this discharge path can take place also if thevoltage for the auxiliary electrode is taken from a, grid having apositive voltage in the other discharge path. Preferably, the auxiliaryelectrode and the grid having a positive voltage are chosen in such amanner that the influence of the auxiliary electrodeon the associatedcontrol grid is greater than the influence of the grid having a positivevoltage on the control grid in question.

In order that the invention may be clearly understood and readilycarried into effect it will now be set out more fully with reference tothe accompanying drawing given by way of example.

The drawing represents a circuit arrangement for amplifyin low frequencyoscillations, comprising two dischai'gepaths both of which are mountedina single discharge tube 5. This tube comprises a control grid 2, ascreen grid consisting of two parts 3, 3, a suppressor grid 4, an anode5 anda cathode is which is connected to the suppressorgrid i. Theaforesaid auxiliary electrode is constituted bythe part 3 of the screengrid. The discharge path comprising this auxiliary electrode iscontrolled by a part of the control grid having an uneven winding pitch,the control grid portion located in the other discharge path having aconstant winding pitch. The two control grid parts are directlyconnected electrically. The suppressor grid 6, the anode 5, and thecathode 6 serve simultaneously for the two discharge paths.

The screen grid part 3 is connected, through a resistancet, to thepositive terminal of a source of direct voltage (not represented) andthrough a condenser 9 to the cathode 6.

According to theinvention the auxiliary electrocle 3' islconnected tothe screen grid portion 3 through 'a'preferably high resistance it.Moreover, the auxiliary electrode is connected to thecathodeffifthrougha condenser H. The anode 3 circuit includes acomparatively high resistance I2.

The low frequency oscillations to be amplified are sup-plied, jointlywith a control Voltage depending upon theaniplitude of theseoscillations, to the control grid 2. The amplified oscillations can betaken from the resistance l2 through the intermediary of a condenser l3.

The circuit arrangement yields the same effect as that according to thecopending patent appli cation, heretofore mentioned, i. e. a highmaximum, noiseless amplification (with a low anode current) if theamplitude of the oscillations to be amplified is comparatively small,and an amplification which is Well regulable if this amplitude iscomparatively large. The high maximum amplification is obtained by meansof the discharge path comprising the screen grid part 3, the regulableamplification being obtained by means of the discharge path comprisingthe auxiliary electrode 3".

However, the automatic changing over from one discharge path to theother takes place, at least partly, in a manner which is different fromthe aforesaid constructional form of the circuit described in thecopending patent application, heretofore mentioned. With a largeamplitude of the oscillations to be amplified, consequently at a highnegative value of the control voltage, the right-hand discharge path iscut ofi by the control voltage supplied to the control grid 2, whereasthe left-hand discharge path keeps functioning. With a small amplitudeof the oscillations to be amplified, and consequently at a low negativevalue of the control voltage, the right-hand discharge path keepsfunctioning, Whereas the left-hand discharge path is entirely or almostentirely blocked. This blocking takes place as follows: In the case of adecrease in negative value for the control voltage the current of thescreen grid 3 increases, with the result that the positive voltage ofthis grid and consequently also the voltage of the auxiliary electrode3' decreases, since the increased screengrid current contributes to thevoltage loss through the resistance 8. However, the voltage of theauxiliary electrode 3' assumes a value which is even smaller than thevoltage of the screengrid 3, because the current flowing to theauxiliary electrode brings about a voltage loss through the highresistance l0. Since the influence exerted by the auxiliary electrode inthe control grid part having, an uneven Winding pitch, is greater thanthe influence of the screen grid in the control-grid part having aconstant winding pitch, the voltage drop of the auxiliary electrodeinvolves a decrease in current strength of the left-hand discharge pathand consequently of the current flowing to the auxiliary electrode. By asuitable choice of the resistances 8 and I it is ensured that thevoltage of the auxiliary electrode 3' drops to such a degree that thedischarge path in question practically no longer contributes to thecurrent and consequently to the amplification.

In a practical construction of the present arrangement the resistance 8has a value of 1 megohm, the resistance It] having a value of 1.5megohm. The influence of the auxiliary electrode 3 on the control grid 2exceeds that of the screen grid 3 on the control grid 2 (about 0.1 and0.02 r spectively). In the following table the values of the screen-gridcurrent I3 of the current I3 flowing to the auxiliary electrode, of thescreen grid voltage V3 and of the voltage of the auxil- 4 iary electrodeV3 are stated for two values of the control voltage (viz. 0 volt and 5volts). Control voltage=0 volt.

n=3o0 milliamperes I3 =50 milliamperes Control voltage- 5 volts I3=0milliamperes V3=3l0 volts I3 =l4=0 milliamperes V3 volts From this tableclearly appears that in the controlled state (control voltage=5 volts)exclusively the left-hand discharge path conveys current, Whereas theright-hand discharge path is cut off, and that in the non-controlledstate (control voltage;0, volt) the amplification mainly takes place bythe right-hand discharge path, the left-hand discharge path beingsubstantially entirely cut out.

The present circuit arrangement has the advantage over the arrangementset out in the copending patent application, heretofore mentioned, thatthe anode impedance may have a high value, since the resistance I0 is nolonger connected tothe anode 5.

What I claim is:

1. An amplifying circuit arrangement to couple a source of desiredsignals to an output stage comprising, first and second thermionicdischarge paths enclosed in an evacuated envelope, said paths beingconnected in parallel relation and having commoncathodes and anodes, thefirst thermionic discharge path having further a control grid electrodewith given spacing and transconductance and a screen grid electrode, thesecond thermionic discharge path having further a control grid electrodewith larger grid spacing and lower transconductance than the controlgrid of the said first thermionic discharge path and an auxiliaryelectrode between the control grid of the said second thermionicdischarge path and the common anode, means to couple the source ofdesired signals to the cathode-control grid circuits of the said firstand second thermionic discharge paths, a first resistive elementconnected between said auxiliary electrode and said screen gridelectrode, and a second resistive element connecting the said screengrid electrode to a v3=1o0 volts V3'=25 volts source of direct currentpotential; said first and second elements having respective values atwhich said second path is rendered non-conductive when the signalsapplied to said first and second paths attain a relatively low amplitudelevel.

2. An amplifying circuit arrangement comprising a source of desiredsignals, an output stage, first and second thermionic discharge pathsenclosed in an evacuated envelope, said paths being connected inparallel relation and having common cathodes and anodes, the firstthermionic discharge path having further a control grid electrode withgiven spacing, transconductance and constant winding pitch and a screengrid electrode, the second thermionic discharge path having further acontrol grid electrode with larger grid spacing, uneven winding pitchand lower transconductance than the control grid of the said firstthermionic discharge path and an auxiliary electrode between the controlgrid of the said second thermionic discharge path and the common anode,means to couple the control grids of the said first and secondthermionic discharge paths, means to connect said source of desiredsignals to the control grids,

GERRIT HENDRIK PETRUS ALMA.

REFERENCES CITED The following references are of record in the file ofthis patent:

Number UNITED STATES PATENTS Name Date Black Sept. 3, 1935 BallantineDec. 29, 1936 Loughren Nov. 25, 1941 Boucke Mar. 30, 1943 Deerhake Jan.30, 1945

